This invention relates generally to the production of articles from low density paper and paperboard and to insulated articles made therefrom, and in particular, relates to cups made of low density paper and paperboard.
Insulated cups and containers are widely used for serving hot and cold beverages and other food items. Such articles may be made from a variety of materials including polystyrene foam, double-walled containers, and multi-layered paper-based containers such as paperboard containers containing an outer foamed layer. Paper-based containers are often more desirable than containers made from styrene-based materials because paper-based materials are generally more amenable to recycling, are biodegradable and have a surface more acceptable to printing. However, multi-layered and multi-walled paper-based containers are relatively expensive to manufacture compared to polystyrene foam-based articles and often do not exhibit comparable insulative properties. Paperboard containers having an outer foam insulation layer are generally less expensive to produce than double-walled containers, but the outer surface is less compatible with printing.
Attempts have been made to improve certain properties of paper by incorporating expanded as well as unexpanded microspheres within the paper. For example, U.S. Pat. No. 3,556,934 to Meyer describes production of paper products for books, magazines, and the like wherein unexpanded microspheres are incorporated into a papermaking furnish which is then formed into a web and dried. The microspheres expand on drying to produce a sheet said to have improved stiffness and caliper. However, the ""934 patent deals with relatively low basis weight paper not suitable for insulated container manufacture, makes no mention of use of the product in the manufacture of paperboard containers having insulative properties, and gives no teaching as to how such a product could be produced so as to enable use of the product in fabricating insulative containers such as cups and the like.
Accordingly, there continues to be a need for paper-based materials which have good insulative properties and which can be produced on a competitive basis with polystyrene foam-based articles.
The present invention is directed to a low density paperboard material for use in producing insulated containers such as paper cups. In general, the paperboard material comprises a paperboard web that includes expanded microspheres and has a basis weight suitable for manufacturing an insulated container such as a paper cup, in which case the board preferably has a basis weight ranging from about 200 to about 220 lbs/3000 ft.2 (3MSF). Low density paperboard according to the invention incorporates from about 0.25 to 10 wt. % (on a dry basis) expanded micropheres and has a relatively low apparent density ranging from about 6.0 to about 10 lb./3MSF/mil and a relatively high caliper ranging from about 24 to about 35 mil. These properties are especially well-suited for board products used to manufacture cups, particularly cups dimensioned to contain 16 ounces of fluid (Internal base diameter=2xc2xc inches). However, it is to be appreciated that low density paperboard according to the invention may find utility in a wide range of applications and product dimensions where properties of low density/thermal insulation are desirable.
In cup applications where the product is intended to contain a liquid, it is preferred to include on the surface of the board to contact the liquid a barrier coating suitable for blocking passage of liquid into the board. A low density polyethylene coating is preferred for this purpose.
For cups and containers intended for heated fluids, it is generally only necessary to coat the surface of the board to be used on the inside of the container, and for chilled fluids (i.e. iced or cold drinks) where outer condensation is an issue, to coat both surfaces.
For paperboard according to the invention within the aforementioned ranges of density and caliper destined for cup manufacture, it is preferred that the board also be formed so as to exhibit an average (i.e. average of MD and CD) internal bond strength of at least about 100xc3x9710xe2x88x923 ft-lbf. This minimum internal bond together with other board properties is believed necessary in order that the board may be successfully converted into cup shapes and similar articles without significant adverse effects caused by the converting operations. Among these adverse effects are so-called xe2x80x9cbucklesxe2x80x9d which can appear along the height of a cup during the process of cup forming where polyethylene-coated board develops small ripple-like deformations as a blank is wrapped around a mandrel to form a cup wall.
Other factors believed to influence development of buckles during conversion operations include the method of applying the coating onto the board and the weight of the coating. Thus, for conventional extruded polyethylene coating conditions (speed and weight) the 100xc3x9710xe2x88x923 ft-lbf minimum average internal bond is believed necessary for proper conversion, while lowering the extrusion speed by 25 percent below the conventional speed or increasing the coat weight in the neighborhood of about 50 percent above the conventional weight will ordinarily allow a corresponding reduction in the minimum average internal bond to about 80xc3x9710xe2x88x923 ft-lbf.
According to one aspect of the invention, the uncoated low density board surface has a roughness substantially higher than conventional cupstock on the Sheffield smoothness scale which, quite surprisingly, results in comparable print quality in a flexo printing operation. Thus, for a typical low density board according to the invention suitable for cupmaking, the uncoated surface of the board exhibits a Sheffield smoothness of at least about 300 SU and a PPS10 smoothness at or below about 6.5 microns.
The low density board of the invention is contrasted with conventional cupstock which is calendered to provide, among other things, a much higher density in the order of 11-12 lb/3MSF/mil, a much lower caliper in the range of 20 mil, and an associated relatively smooth surface in the range of from about 160 to about 200 SU believed necessary for acceptable print quality. This higher density/lower caliper board has the effect of increasing the thermal conductivity of the board (i.e., decreased insulation)
In another aspect, the invention provides a method for making a low density paperboard material suitable for use in producing insulated containers such as cups. The method includes providing a papermaking furnish containing cellulosic fibers, and from about 0.25 to about 10% by weight dry basis expandable microspheres, preferably from about 5 to about 7 wt. %, forming a paperboard web from the papermaking furnish on a papermaking machine, and drying and calendering the web to an apparent density ranging from about 6.0 to about 10.0 lb/3MSF/mil, most preferably from about 6.5 to about 10.0 lb/3MSF/mil, and a caliper of from about 24 to about 35 mil, most preferably from about 28 to about 35 mil.
In yet another aspect, the invention provides a method for making an insulated container such as a paper cup from a paperboard material. The method includes providing a papermaking furnish containing cellulosic fibers and from about 0.25 to about 10 wt % dry basis expandable microspheres, preferably from about 5 to about 7% by weight, forming a paperboard web from the papermaking furnish on a paper machine, and drying and calendering the web to an apparent density ranging from about 6.0 to about 10.0 lb/3MSF/mil, preferably about 6.5 to about 10.0 lb/3MSF/mil, a caliper ranging from about 24 to about 35 mil, preferably from about 28 to about 35 mil, an internal bond of at least about 80xc3x9710xe2x88x923 ft-lbf, preferably at least about 100xc3x9710xe2x88x923 ft-lbf, and a Sheffield smoothness of at or above about 300 SU, and thereafter forming the web into a container such as a paper cup including the paperboard web at least for the sidewall portion of the cup.
Paperboard webs made according to the invention exhibit increased insulative properties compared to conventional single ply paperboard webs and are significantly less expensive to produce than multi-layered paperboard products or paperboard products containing a foamed outer coating. The low density paperboard material may therefore be converted into cups and other insulated containers on conventional processing equipment with minimal loss in machine speed, and a reduced tendency to form buckles and other irregularities in the converting operations.
A key feature of the invention is the use of expandable microspheres in the papermaking furnish and a resulting relatively low density/high caliper board containing the expanded spheres. Although the presence of microspheres in the papermaking furnish had been thought to adversely effect physical properties of the resulting materials for certain end use applications, it has now been found that by producing the materials according to the invention, the resulting board may be readily converted into containers such as insulated cups. Without desiring to be bound by theory, it is believed that suitable insulative paperboard products having strength properties required for cup converting operations may be produced by significantly increasing the caliper of the material and decreasing the density (compared to conventional board products) while maintaining a relatively high internal bond.